While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and enabled devices to operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
EL devices that emit white light have proven to be very useful. They can be used with color filters to produce full-color display devices. They can also be used with color filters in other multicolor or functional-color display devices. White EL devices for use in such display devices are easy to manufacture, and they produce reliable white light in each pixel of the displays. Although the OLEDs are referred to as white, they can appear white or off-white, for this application, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. Thus there is a need for new materials that provide high luminance intensity for use in white OLED devices.
One of the most common materials used in many OLED devices is tris(8-quinolinolato)aluminum (III) (Alq). This metal complex is an excellent electron-transporting material and has been used for many years in the industry. However, it would be desirable to find new materials to replace Alq that would afford further improvements in electroluminescent device performance.
Many new organometallic materials have been investigated for use in electroluminescent devices. For example, U.S. Pat. No. 6,420,057 and JP 2001/081453 describe organometallic complexes included in a light-emitting layer. These complexes include a metal-nitrogen ionic bond as well as a metal-nitrogen dative or coordinate bond. US 2003/068528 and US 2003/059647 describe similar materials used as blocking layers and hole-transporting layers respectively. JP 09003447 reports related organometallic complexes as useful electron-transporting materials.
Commonly assigned U.S. patent application Ser. No. 11/172,338 filed Jun. 30, 2005, (publication No. 2007/0003786)describes an EL device containing a layer that does not emit light and included in that layer is a metal complex that can provide desirable electroluminescent properties. Commonly assigned U.S. patent application Ser. No. 11/334,532 filed Jan. 18, 2006, (publication No. 2007/0166566) describes an EL device containing a layer including a metal gallium complex and a layer that includes an alkaline metal material that also can provide desirable electroluminescent properties.
US 2005/0179370 describes EL devices having more than one electron-transporting layer wherein the layers have different electron-transporting properties. It is reported that it is preferable for the cathode-side electron transporting layer to have an energy gap that is the same as or greater than the adjacent (anode-side) electron-transporting layer. Thus, material in the cathode-side ETL would have an equal or higher (more positive) lowest-unoccupied molecular orbital (LUMO) energy level relative to material in the anode-side ETL, as shown in FIG. 5 of US 2005/0179370. However, this may not result in the most desirable electroluminescent properties. Thus, despite these improvements there remains a further need for combinations of materials that can offer enhanced luminance, reduced drive voltage, or improved stability or all of these features.